The present invention relates to pressure-sensitive adhesives comprising an aqueous polymer dispersion, wherein the polymer is composed of
from 50 to 99.9% by weight of at least one C1-C12 alkyl (meth)acrylate (monomers a)
from 0.05 to 20% by weight of at least one vinylaromatic compound (monomers b)
from 0.05 to 10% by weight of at least one ethylenically unsaturated hydroxy compound (monomers c)
from 0 to 10% by weight of an ethylenically unsaturated acid or an acid anhydride (monomers d) and
from 0 to 30% by weight of other ethylenically unsaturated compounds (monomers e)
the weight percentages being based on the polymer, and the polymer dispersion comprising an emulsifier consisting to the extent of at least 5% by weight of aromatic carbon atoms (and called aromatic emulsifier for short) or an emulsifier mixture consisting to the extent of at least 10% by weight of an aromatic emulsifier.
Pressure-sensitive adhesives (PSAs) form a permanently tacky film which adheres to a very wide variety of surfaces even under slight pressure at room temperature. Pressure-sensitive adhesives are used to produce self-adhesive products such as labels, tapes and films. Products of this kind are very easy to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are required. Moreover, there is no xe2x80x9copen timexe2x80x9d within which the adhesive bond must be made. The quality of a self-adhesive article depends essentially on whether the cohesion (internal strength of the film of adhesive) and its adhesion (to the surface that is to be bonded) are in tune with one another in accordance with the application.
In the case of pressure-sensitive adhesives for sheets, adhesive tapes or labels, in particular, the level of cohesion must be sufficient for no stringing and no emergence of glue at the edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be at a high level in order to provide good sticking on the substrate that is to be bonded.
In general, it is impossible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties, or at least maintain one property unchanged while improving the other.
Adhesives, including pressure-sensitive adhesives, based on polyacrylate dispersions are known, for example, from WO 98/06763, WO 98/23656 or EP-A-625 557. Copolymers of this kind still do not have the desired level of adhesion and cohesion or the desired ratio of the one to the other. DE 19818394 (OZ48974) discloses pressure-sensitive adhesives with an aromatic emulsifier.
It is an object of the present invention to provide pressure-sensitive adhesives having improved adhesion and/or cohesion.
We have found that this object is achieved by the pressure-sensitive adhesives defined above.
The polymer of the aqueous polymer dispersion is composed of the monomers a) to e) defined at the outset.
The monomers a) comprise a C1-C12 alkyl (meth)acrylate, including in particular mixtures of the alkyl (meth)acrylates.
Suitable preferably are C1-C8-alkyl (meth)acrylates.
Suitable examples are methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate and ethyl acrylate.
Monomers b) comprise preferably xcex1-methylstyrene or styrene.
Particular preference is given to styrene. The proportion of monomers b) in the polymer is preferably not more than 10%, in particular not more than 5% and, with particular preference, not more than 3% by weight, and is preferably more than 0.1% by weight, in particular more than 0.5.
Monomers c) are preferably C1-C18 hydroxyalkyl (meth)acrylates.
Particular preference is given to C2 to C8 hydroxyalkyl (meth)acrylates. Suitable examples are hydroxypropyl acrylate and methacrylate, and hydroxy-n-butyl acrylate and methacrylate.
Hydroxypropyl acrylate or hydroxypropyl methacrylate is particularly preferred.
The proportion of the monomers c) in the polymer is preferably up to 10%, in particular 5% and, with particular preference, 3% by weight, preferably at least 0.1% by weight and, in particular, at least 0.5% by weight.
Monomers d) are, for example, ethylenically unsaturated monomers having in particular carboxyl groups, such as (meth)acrylic acid, maleic acid, ethylenically unsaturated acid anhydrides or monoesters, such as maleic anhydride or maleic or fumaric monoesters. Acrylic and methacrylic acid are preferred. The proportion of monomers d) is preferably not more than 5% by weight, based on the polymer.
Further monomers, e), can be of any kind. Preferred examples which may be mentioned are vinyl esters, olefins, diolefins, ethylenically unsaturated amides or nitriles etc.
The polymer consists preferably of
and with particular preference of
The glass transition temperature of the polymer can be determined by customary methods, such as differential thermal analysis or differential scanning calorimetry (see e.g. ASTM 3418/82, midpoint temperature).
The glass transition temperature (Tg) of the polymer is preferably from xe2x88x9260xc2x0 C. to +10xc2x0 C., more preferably from xe2x88x9250xc2x0 C. to xe2x88x9210xc2x0 C. and, with very particular preference, from xe2x88x9250xc2x0 C. to xe2x88x9220xc2x0 C.
The polymer is preferably prepared by emulsion polymerization and is therefore an emulsion polymer.
In emulsion polymerization it is common to use ionic and/or nonionic emulsifiers and/or protective colloids, or stabilizers, as surface-active compounds.
In the present case use is made in accordance with the invention of an emulsifier consisting to the extent of at least 5% by weight of aromatic carbon atoms (called aromatic emulsifier for short) or an emulsifier mixture which comprises at least 10% by weight of such an aromatic emulsifier, based on the overall amount of emulsifier.
The aromatic emulsifier preferably consists to the extent of at least 10% by weight, with particular preference at least 20% by weight, of aromatic carbon atoms (by which are meant carbon atoms that are part of an aromatic ring system).
The proportion of aromatic carbon atoms is generally below 80% by weight.
The aromatic emulsifier is preferably an ionic emulsifier, especially one having one or two, preferably two, sulfate groups.
The molar weight of the emulsifier is preferably less than 2000 and especially less than 1000 g/mol.
With particular preference, the aromatic emulsifier comprises an ionic compound having a molecular weight of less than 2000 g/mol and comprising at least one sulfate group or sulfonate groups, preferably two sulfate groups or sulfonate groups with two unsubstituted or substituted phenyl groups.
Preference is given to aromatic emulsifiers having one or two, preferably two, sulfonate groups.
The emulsifier particularly preferably comprises a compound of the formula 
where X is O, S, CH2, NH or NR7, one or two of R1 to R6 are SO3xe2x8ax96K⊕and the others of R1 to R6 are H or C1-C18 alkyl, R7 is C1-C18 alkyl and K is a countercation.
X is preferably O. Preferably, one or two of R1 to R6 are C1-C18 alkyl, especially C6-C18 alkyl, and the others of R1 to R6 are hydrogen atoms and the sulfonate groups.
K is preferably a countercation selected, for example, from the alkali metals, ammonium and hydrogen. Sodium is particularly preferred. Compounds of the formula I normally also comprise a mixture of compounds having different degrees of substitution (mono- or dialkylated) and different positions of the substituents (of the sulfonate groups and of the one or two alkyl groups). Compounds having the formula I are marketed under the tradename Dowfax(copyright) 2A by Dow Chemical Company.
Relative to the use of an aromatic emulsifier alone, the use of a mixture comprising an aromatic emulsifier and an emulsifier without aromatic carbon atoms (nonaromatic emulsifier) is preferred.
The nonaromatic emulsifier is preferably of the formula
R8xe2x80x94Oxe2x80x94(Zxe2x80x94O)nxe2x80x94SOxe2x88x923K+xe2x80x83xe2x80x83II
where:
R8 is C1-C18 alkyl, preferably C10-C16 
Z is CH2xe2x80x94CH2 or 
preferably CH2xe2x80x94CH2 
n is an integer from 1 to 40, preferably from 2 to 30
K+ is a cation, e.g. H+, an alkali metal cation of e.g. Na or K, or ammonium.
Compounds of the formula II are intended to embrace compounds comprising both 
Compounds of the formula II are marketed, for example, under the designation Disponil(copyright) FES 77 by Henkel.
The emulsifier mixture consists preferably of
from 20 to 95% by weight of an aromatic emulsifier, especially that of the formula I, and
from 5 to 80% by weight of a nonaromatic emulsifier, especially that of the formula II.
The proportion of the aromatic emulsifier is preferably from 30 to 95% by weight and that of the nonaromatic emulsifier from 5 to 70% by weight, the weight percentages being based on the overall amount of the emulsifiers.
The emulsifier or emulsifier mixture is normally used in amounts of from 0.1 to 15% by weight, preferably from 0.3 to 5% by weight and, with particular preference, from 0.5 to 3% by weight, based on the monomers to be polymerized. Accordingly, the resulting polymer dispersion includes the stated amounts of emulsifier.
Examples of water-soluble initiators for the emulsion polymerization are ammonium salts and alkali metal salts of peroxodisulfuric acid, such as sodium peroxodisulfate, hydrogen peroxide, or organic peroxides, such as tert-butyl hydroperoxide.
Particular suitability is possessed by what are known as reduction/oxidation (redox) initiator systems.
The redox initiator systems consist of at least one reducing agent, which is usually inorganic, and one organic or inorganic oxidizing agent.
The oxidizing component comprises, for example, the abovementioned emulsion polymerization initiators.
Reducing components comprise, for example, alkali metal salts of sulfurous acid, such as sodium sulfite, sodium hydrogen sulfite, alkali metal salts of disulfurous acid, such as sodium disulfite, bisulfite adducts with aliphatic aldehydes and ketones, such as acetone bisulfite, or reducing agents such as hydroxymethanesulfinic acid and its salts, or ascorbic acid. The redox initiator systems can be used along with soluble metal compounds whose metallic component is able to exist in a plurality of valence states.
Examples of customary redox initiator systems are ascorbic acid/iron (II) sulfate/sodium peroxodisulfate, tert-butyl hydroperoxide/sodium disulfite, tert-butyl hydroperoxide/sodium hydroxymethanesulfinate. The individual components, such as the reducing component, can also comprise mixtures; for example, a mixture of sodium hydroxymethanesulfinate and sodium disulfite.
Said compounds are generally employed in the form of aqueous solutions, the lower concentration being determined by the amount of water which is acceptable in the dispersion and the upper concentration by the solubility of the relevant compound in water. The concentration is generally from 0.1 to 30% by weight, preferably from 0.5 to 20% by weight and, with particular preference, from 1.0 to 10% by weight, based on the solution.
The amount of the initiators is generally from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, based on the monomers to be polymerized. It is also possible to use two or more different initiators in the course of the emulsion polymerization.
Regulators as well can also be employed for the polymerization, and reduce the molecular mass. Examples of suitable compounds are those having a thiol group, such as tert-butyl mercaptan, ethylhexyl thioglycolate, mercaptoethanol, mercaptopropyl-trimethoxysilane or tert-dodecyl mercaptan. The proportion of these regulators can in particular be from 0 to 0.3% by weight, preferably from 0.02 to 0.3% by weight, based on the polymer.
The emulsion polymerization is generally carried out at from 30 to 100xc2x0 C., preferably from 50 to 95xc2x0 C. The polymerization medium can consist either of water alone or of mixtures of water with water-miscible liquids such as methanol. Use is preferably made of water alone. The emulsion polymerization can be conducted either as a batch process or in the form of a feed process, including staged or gradient procedures. Preference is given to the feed process, in which a portion of the polymerization mixture is introduced as initial charge, heated to the polymerization temperature and initially polymerized, and then the remainder of the polymerization mixture is supplied to the polymerization zone continuously, in stages or under a concentration gradient and normally by way of two or more spatially separate feed streams of which one or more comprise the monomers in pure or in emulsified form, this supply taking place with the polymerization being maintained.
The manner in which the initiator is added to the polymerization vessel in the course of free-radical aqueous emulsion polymerization is known to the skilled worker. It can either be introduced entirely in the initial charge to the polymerization vessel or else inserted continuously or in stages at the rate at which it is consumed in the course of the free-radical aqueous emulsion polymerization. In each individual case it will depend in a manner known to the skilled worker both on the chemical nature of the initiator system and on the polymerization temperature. It is preferred to introduce one portion in the initial charge and to supply the remainder to the polymerization zone at the rate at which it is consumed.
In order to remove the residual monomers it is also customary, following the end of the actual emulsion polymerization, i.e. after a monomer conversion of at least 95%, to add initiator.
In the case of the feed process, the individual components can be added to the reactor from above, through the side or from below, through the reactor floor.
The emulsion polymerization gives aqueous dispersions of the polymer, generally having solids contents of from 15 to 75% by weight and, preferably, from 40 to 75% by weight.
For a high space/time yield of the reactor, dispersions having a very high solids content are preferred. In order to be able to obtain solids contents  greater than 60% by weight it is advisable to establish a bimodal or polymodal particle size, since otherwise the viscosity becomes too high and the dispersion can no longer be handled. Producing a new generation of particles can be carried out, for example, by adding seed (EP 81083), by adding excess emulsifier, or by adding miniemulsions. A further advantage associated with the low viscosity at high solids content is the improved coating behavior at high solids contents. The production of (a) new particle generation(s) can be made at any desired point in time. It is guided by the target particle-size distribution for low viscosity.
The resulting polymers or polymer dispersions are used as pressure-sensitive adhesives. They are particularly suitable as PSAs for labels or adhesive tapes.
The polymers are preferably used in the form of their aqueous dispersion. The aqueous polymer dispersions can be used as pressure-sensitive adhesives without further additives.
In the case of use as pressure-sensitive adhesive it is possible to add to the polymers or to the aqueous polymer dispersions a tackifier, i.e. a tackifying resin. Tackifiers are known, for example, from Adhesives Age, July 1987, pages 19-23 or Polym. Mater. Sci. Eng. 61 (1989), pages 588-592.
Examples of tackifiers are natural resins, such as rosins and their derivatives by disproportionation or isomerization, polymerization, dimerization or hydrogenation. They can be in their salt form (with, for example, mono- or polyvalent counterions (cations)) or, preferably, in their esterified form. Alcohols used for esterification can be mono- or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1,2,3-propanethiol and pentaerythritol.
Also used, furthermore, are hydrocarbon resins, examples being indene-coumarone resins, polyterpene resins, hydrocarbon resins based on unsaturated CH compounds, such as butadiene, pentene, methylbutene, isoprene, piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene, cyclohexadiene, styrene, xcex1-methylstyrene, vinyltoluene.
Further compounds increasingly being used as tackifiers are polyacrylates of low molecular weight. These polyacrylates preferably have a weight-average molecular weight MW of less than 30,000. The polyacrylates consist preferably to the extent of at least 60% by weight, in particular at least 80% by weight, of C1-C8 alkyl (meth)acrylates.
Preferred tackifiers are naturally occurring or chemically modified rosins. Rosins consist predominantly of abietic acid or its derivatives.
The tackifiers can be added in a simple manner to the polymers of the invention, preferably to the aqueous dispersions of the polymers. The tackifiers are preferably themselves in the form of an aqueous dispersion in this case.
The proportion of the tackifiers is preferably from 5 to 100 parts by weight, with particular preference 10 to 50 parts by weight, based on 100 parts by weight of polymer (solids/solids).
In addition to tackifiers it is also possible, for example, for other additives, such as thickeners, antifoams, plasticizers, pigments, wetting agents or fillers, to be employed in the case of utility as a pressure-sensitive adhesive. The pressure-sensitive adhesives of the invention may therefore also comprise not only the aqueous polymer dispersion but also tackifiers and/or the above additives.
The pressure-sensitive adhesives can be applied by customary methods, such as by rolling, knifecoating, brushing, etc., to substrates, examples being paper or polymer films consisting preferably of polyethylene, polypropylene, which can be biaxially or monoaxially oriented, polyethylene terephthalate, polyvinyl chloride, polystyrene, polyamide or metal. The water can be removed preferably by drying at from 50 to 150xc2x0 C. For subsequent use, the PSA-coated side of the substrates, e.g. the labels, can be covered with a release paper, such as with a siliconized paper.
The pressure-sensitive adhesives of the invention exhibit 30 improved adhesion and cohesion and/or an improved adhesion/cohesion ratio.